Cathode-ray tube



Nov. 2, 1948. WEIMER 2,452,619

CATHODE-RAY TUBE Filed Feb. 7, 1946 INVENTOR.

} 2270/ K nl mer Patented Nov. 2, 1948 CATHODE-RAY TUBE Paul K. Weimer,Princeton, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application February 7, 1946, Serial No. 646,076

2 Claims.

'This invention relates to cathode ray tubes for television and otherpurposes of the orthicon type disclosed in my application, filedNovember 28,

1945, Serial No. 631,440.

means is disclosed for preventing helical motion In that applicationscanning coil in respect to the decelerating ring around the spaceimmediately in front of the target. This adjustment varied the relativephases of the spiraling effect of the coil and decelerating ring so thatthey could be brought into section, partly diagrammatic, of a cathoderay tube involving the improvement.

Referring to the drawing, the tube has an evacuated container l usuallyof glass, with the usual indirectly heated cathode 2, grid 3 and firstanode l constituting the gun. The end 5 of the anode also constitutesthe first dynode of the multiplier. This may be a thin sheet ofsilver-magnesium alloy clamped over or otherwise fastened to the end ofthe anode, which may be of non-magnetic *stainless steel. Around the gunin spaced or insulated relation are positioned the other multipliersconsisting, as shown, of the second stage 1, third stage 8,;iourth stage9 and fifth and last stage l0. "Between the fourth and fifth stages islocated a fine mesh screen H constituting the collector electrode forthe electrons bombarded from the last electrode. The second, third andfourth stages have a dynode having radial slanting blades I2, somewhatresembling an electric fan. Each of these three stages has a. fine meshscreen I3 attached to the rim of the support for the dynodes and spaceda small distance from the dynode blades, which may be made of silvermagnesium alloy, for example. The particular form of multiplier is notclaimed herein, but is fully described and claimed in my co-pendingapplication, filed September 16, 1944, Serial No. 554,494, now U. S.Patent No. 2,433,941, issued June 6, 1948, and

opposition. The result was the helical motion reference is made theretofor adetail explanation produced by the two sources was made to cancel,oi the multiplier stages,

so that the beam had no helical motion when it Before describing theparticular improvement arrived at the target. In that application it wasclaimed in this application, it may be said that also disclosed that thedecelerating lens could be the return beam bombards secondary electronsmadeto have less spiraling effect on the electrons 5 from the dynode o hfi Stage 5 and these by using a fine mesh screen close to the target aredirected into the field of the dynode and on the gun side thereof. Thepresent application screen of the second stage, passing through the isdirected to the latter modification of that prior fine .mesh screen andbombarding secondary application. electrons from the dynode blades.These sec- I It is an object of this invention to provide a 71 ondariesare attracted to the higher potential cathode ray tube deceleratingmeans that will field of the next stage 8, and so on. The secgive lessscanning of the return beam over the ondaries bombarded from the dynodeblades of first dynode of the multiplier, which reduces mul the fourthstage are attracted through the meshes p Shadingof the collector screeni l and bombard secondaries It is another object of the invention toconstruct from the dynode of the fifth stage III, which is a a tube withthe decelerating electrode adjacent fiat metal disc of silver magnesium.The secthe scanned surface of the target to provide for ondariesemittedby this last dynode are attracted more uniform landing of theelectrons from the to the collector screen and pass out of the tube beamacross the raster. to the pre-amplifier or any other desired device.Another object of the invention is to provide The tube has an electrodeI4, commonly called a higher electric field at the target surface to thepersuader, which establishes a certain field improve the resolution.that directs the secondaries from the first dynode Other objects of theinvention will appear in 5 into the second stage. the followingspecification, reference being had j The oa d Wall coating y, in fact.be a to the drawing, in which the single figure is a 40 m t l coating onh inside f the env lope l, but

I prefer to make it of non-magnetic metal of relatively poorconductivity, such as Nichrome cylinder l5. Opposite the end of cylinderl5 remote fromthe gun is placed a target I6 consisting, of a thinsemi-conducting sheet of glass, described and claimed in the applicationof Albert Rose, filed November 28, 1945, Serial No. 631,441. Adjacentthe unscanned surface of the glass target IE is a fine mesh screen I!which may be contact spaced from the target, that is, touching the same,or it may be spaced to 500 mils orniorefas "disclosed in'the applicationof Harold B. Iiaw-,-filed April 19, 1945,Serial No. 589,241. The target16 and screen I! are suitably mounted "in a metal ring 'or cylinderframe IB.

thereto. .sulated relation to the other electrodes,.but I find On theinside surface of the end of the envelope remote from the gun ispositioned the photocathode l9, marked PC in the drawing, which may be athin metal coating on the inside wall that is substantially transparentto light and yet is sufficiently conducting for movement of electronsthereover. Adjacent the photocathode i9 is an electrode "20, .called aphotocathode ring, marked "PCRX in-the drawing, and positioned. betweenring 20 and the target frame I8 is another ring 2| conductively joinedto the frame Hi. All

the parts and electrodes inside the tube, except photocathode H], asshown, may be supported in any way known in the art, forexample, asdisclosed in the applications {of Stanley V. .Forgue, filed February 7,1946, Serial No. 646,075, and file-d January 28, 1946, Serial No.643,925, and now D. S.

Patent No. 2,441,315, issued May '11, 1948. My invention is notdependent upon the particular supporting means and hence it is notshown.

Around the outside of the tube, is arranged a .solenoid coil .22 adaptedto produce a uniform direct current magneticfocusing field for the out-;going and returning beam electrons, as well as for the photoelectronsemittedby the photocathode 19. Outside the tube is also arranged a com-;pensating coil 23, as disclosed and claimed in the application of.Albert Rose, filed April 11, 1942,

Serial No. 438,562, nowU- S. Patent No. 2,407,905,

.issued-;September 17, 1946. Thiscoil produces a :fieldat a-right angleto the axis of the tube and, upon adjustment around the axis, iteliminates helical motion of the beam electrons due to the unavoidableslight misalignment of the gun in factory construction. Inside the coil22 is positioned the horizontal and vertical deflecting coils, shown asa unit. As is well known, one-coil of the unit produces a deflecting.field at a right .angle to the-axis of the tube to produce line scan-.sion of the target by the beam and the other produces a field ata rightangle to the axis and .also to the field .of the other deflecting coilto produce frame scansion. The line scansion coil will beconnectedto asaw-tooth generator ofline .scansion frequency and-the other one will beconnected to a saw-tooth generator of frame scanning frequency. Thesesaw-tooth generators are .well known in theart and are not shown.

-To provide the more uniform fieldadjacent the raster surface of thetarget l6, afine mesh screen 2.5 isarranged in front of the target andparallel This screen vmay 'be supported in inthatpexcellent results areobtained when it is mounted in a metal framefitting conductively insidethe cylinder electrode 15, corresponding to target isimportant andshould satisfy the follow- .ing. conditions:

.1. Itf-Should be out of focus for the. incident and :alsothe returnbeam.

, ..2. It should not introducespurious patterns.

The first condition is not critical. The screen vis atits mostout-of-focus position when the distanceof the screen jfromthe. target incentimeters is equal approximately to :where E is the screen potentialin volts andis the :magnetic field -in gausses produced .by the magneticfocusing field of coil. 22.. The second condition is more critical It11.

found experimentally that occasionally an apparent screen pattern can beseen even when the first condition is completely satisfied. This is dueto an interference between the screen shadow pattern impressed on theout-of-focus incident beam and the similar pattern impressed on theout-of-focus return beam. The type of spurious screen pattern 'seendepends upon. the transit time of the electrons .between their first andsecond .when mounted directly on thewall cylinder [5 and operated atwall potential, it is slightly advanitageous to have the screen a fewvolts positive with respect to the wall, so that secondary electronsknocked out of the screen by the beam will return thereto and will notreach the multiplier. In this way the noise introduced by the screen canbe minimized.

.The screen 25 should have at least transmission. It should be freeofimperfections and should be ,fine mesh of at least 200 wires per inch.Screens are available to meet-these conditions.

The tube may be operated at various voltages ,in the electrodes, but Ihave indicated on the drawingsatisfactory voltages, positive andnegative with respect to ground. I

Briefly, the operation is as follows:

Alight image focusedon the ph otocathode 1.9 liberates electronstherefrom and these are focused on the glass target by the fieldof coil.22.under the acceleration force of the potential of ring 20 and screenll. Secondary electrons are bombarded from the rear (photocathode-side)sideofthe glass and are collected by screen H, which establishes themaximum potential to which the target may rise. Thus, a positive chargeimage is formed on the rear surface of the glass that exactlycorresponds to the light image focused on thephotocathode I9. As thebeam from the gun scans the front side-0f. the target,.it is deceleratedsubstantially'to zero electron velocity and only enough electrons landon the front side of the target to deposit electrons equivalent to thoseon-the rear side, the potential pattern set up on the front side :by-thecharge image being substantially the same as that "on the rearside, dueto the extremethinnessofthe .glass film, which is about 0.2 milor less.As explained in the said Albert Rose application, Serial No. 631,441,the conductivity of the glass is such that theelectrons-landed passthrough and discharge'the electrostatic image-on the rear side Within aframe time. The remaining electrons of the beam return towards "thegununder the accelerating force of the screen 25 and wall cylinder l5.These returning electrons are focused by the field of coil 22 and landon an area of dynode 5.0utside the aperture through which the beam wasshot from the gun. The returning beam is modulated by loss of electronsto the target l6 and the secondary electrons bombarded from the dynode 5are produced in proportion thereto in multiplied numbers. Theseare-directed by persuader into the "field of-the second multiplier stageand after multiplication by the succeeding stages a greatly multipliedsignal is fed into the output terminal, as already referred to.

The field established at the raster surface of the target is uniformthroughout the raster, except at the perimeter. This can be balanced outby adjustment of the deflecting coils 24, as explained in detail in myfirst-mentioned application. The adjustment is indicated by arrows 26and the adjusting means of that application is diagrammaticallyindicated by the manually adjustable rod 21. Due to the reduced helicaleffect of screen 25 as compared to a decelerating ring, the coils ofdeflecting unit 24 should be designed in size and ampere turns to have ahelical effect as small as possible, so that the helical effects willbalance out to produce uniform landing of the electrons on the target.Since the beam lands electrons on the target in proportion to thevarying potentials established on the elemental areas by the chargeimage, the expression uniform landing is explained as meaning that thenumber of the electron-s landing on the raster at potential +e willalways be the same, no matter whether that area is at the center of theraster or is at the outside or perimeter thereof. To accomplish thisthere must be minimum loss of energy of the electrons due to spiralingand my invention accomplishes the purpose better than deceleratorsheretofore used.

I claim:

1. A cathode ray tube of the Orthicon type comprising a cathode and ananode for forming a beam of electrons, a semi-conducting target, a metalscreen in front of the target, a photocathode and electric fieldproducing means for forming a charge image on one surface of the target,field producing means for scanning electrons from said beam through saidscreen and over the other surface of said target to discharge saidimage, said screen being spaced from the target at such distance as topreclude the superimposing of screen image signals on the beam returningfrom the target through said screen.

2. In cathode ray tubes of the Orthicon type, means comprising a cathodeand an anode for forming a beam of electrons, a semi-conducting target,a fine mesh metal screen in front of the target, a, photo-cathode andelectric field producing means for forming a charge image on one surfaceof the target, field producing means for scanning electrons from saidbeam through said screen and over the other surface of said target todischarge said image, said screen being spaced from the target at suchdistance as to preclude the superimposing of screen image signals on thebeam going toward and returning from the target through said screen.

PAUL K. WEIMER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,021,907 Zworykin Nov. 26, 1935

